This invention relates to a connectionless communication method for the connectionless transmission of user data. More particularly, the invention relates to a datagram-type connectionless communication method in which a cell (also referred to as a xe2x80x9cleading cellxe2x80x9d below) which includes the address of a destination terminal and the like is inserted at the head of a cell stream transferred with an attached prescribed line identifier, and the network transmits a user cell to the destination terminal upon referring to the leading cell.
There are two methods of connecting terminals, namely a connection-oriented method and a connectionless method. As shown in FIG. 31A, the connection-oriented communication method involves dialing a number, for example, verifying the transmitting side (A system) and receiving side (B system) prior to the start of communication so as to ring the other party""s bell, establishing a path between the two sides and then performing data communication via the established path. This method is advantageous in that reliable communication is possible via a connected line and in that it is simple to transfer information after the connection is established. However, since making the connection takes time, this method is not efficient for the transmission of data of short duration.
The connectionless communication method, on the other hand, as shown in FIG. 31B, involves collecting transmission data into packets, attaching destinations to the packets and then sending the packets to a network, whereupon the network checks the destination of each packet one by one and transmits each packet to the destination terminal. Since making a connection with this method does not take time, even short-duration data can be sent efficiently.
In order to make possible multimedia transmission which includes the transmission of video, ATM (Asynchronous Transfer Mode) technology has been developed in association B-ISDNs and a variety of communication services have been realized. A B-ISDN is capable of providing both connection-oriented and connectionless services.
FIG. 32 is a diagram useful in describing connectionless communication in an ATM network. Shown in FIG. 32 are ATM terminals Axcx9cD, ATM exchanges EX1xcx9cEX4, a network NWK composed of a group of ATM exchanges, and a network management center NMC. In a case where terminal A is to perform connection-oriented communication with terminal C, the following procedure for establishing a VCC (Virtual Channel Connection) is required:
(1) Terminal A issues a request for communication with terminal C to the network management center NMC via a common signal line (a VCC for signaling).
(2) The network management center NMC responds to the request from terminal A by performing computation to determine that the VCC between the terminals A and C via the ATM exchanges EX1, EX2, EX3, EX4, in the order mentioned, is the most advantageous route, and by instructing each ATM exchange to establish the VCC.
(3) The ATM exchange EX1 notifies terminal A and the ATM exchange EX4 notifies terminal C of line identifiers VPI, VCI used in communication.
(4) Terminal A disassembles data into cells, attaches the line identifiers of which it has been notified to the cells and then sends the cells to the line. Each of the exchanges EX1xcx9cEX4 exchanges and transmits the cells while changing the line identifiers of the cells. Terminal C accepts the cells, which have the specified line identifiers, transmitted from the exchange EX4 and assembles the cells into data.
(c-1) Problem Which Arises When Connectionless Communication is Used in ATM Networks
The header of an ATM cell contains only virtual line identifiers such as VCI and VPI. That is, the header does not contain an address indicating the destination terminal. Consequently, providing a connectionless communication service such as electronic mail is accompanied with difficulties. For example, if the average downloaded file size on the WWW (Worldwide Web) is two kilobytes and the ATM switch transmits at a rate of 155 megabits per second, then this is the size of a file that can be downloaded in 100 microseconds. However, since an ATM switch requires 10 milliseconds to be connected, 99% is the overhead for the connection. In other words, if an ATM network of the connection-oriented transfer type is used as a transmission path for implementing a connectionless communication service, a delay occurs owing to the time needed to establish the connection.
Solutions to this problem include a PVC (Permanent Virtual Circuit) in the ATM layer and a datagram in a higher layer (e.g., IP layer).
(c-2) PVC Technique
The PVC technique involves establishing end-to-end PVCs beforehand between all of the terminals Axcx9cF that are capable of communicating, as shown in FIG. 33. However, this method is disadvantageous in that PVC resources are wasted. In particular, this method cannot be employed in a large-scale network (the Internet, for example) in which communication is performed between an unspecified number of terminals.
(c-3) Datagram Technique
With the datagram technique, as shown in FIG. 34, PVCs (datagram VCCs: VCC1xcx9cVCC7) are established beforehand between the terminals Axcx9cF and whichever of the exchanges EX1, EX2 is nearest to them, and between the mutually adjacent exchanges EX1 and EX2. An IP packet of a higher layer is assembled from ATM cells by each exchange and switching processing is executed upon observing the terminating-side terminal address that has been entered in the IP packet header. It should be noted that a xe2x80x9cdatagramxe2x80x9d is the unit in which messages are handled in accordance with the IP (Internet Protocol). A datagram technique is a method of providing a destination address in the header of a packet, deciding, on each occasion, the transfer route for each packet and then transmitting the packet.
FIG. 35 is a diagram showing the relationship between an IP packet (IP datagram) and ATM cells. Here P represents an IP packet, which is composed of a header PH and transmission data DT. The header PH includes a variety of information, such as a source address SA and destination address DA. The IP packet P is partitioned into a number of ATM cells CL1xcx9cCLn, and a header HD is added onto each cell at the beginning thereof. Line identifiers (VPI/VCI) included in the headers of the cells CL1xcx9cCLn have identical values.
FIG. 36 is a diagram showing the structure of an exchange which implements the conventional datagram approach. Shown in FIG. 36 are an exchange 1, an ATM switch unit 2, and an IP router 3. (1) A switching able 2a of the ATM switch unit 2 stores the correspondence between the input line identifiers (input VPI/VCI) and output line identifiers (output VPI/VCI), and an ATM switch 2b generally switches a cell, which arrives from the VCC, to a prescribed route based upon the content of the switching table 2a. (2) If a cell arrives from a datagram VCC, the ATM switch unit 2b delivers this cell to the IP router 3. A packet assembler 3a in the IP router 3 assembles ATM cells into an IP packet and delivers the IP packet to a routing controller 3c. (3) A routing table 3b stores the correspondence between terminal addresses and output-destination datagram VCCs. Accordingly, the routing controller 3c refers to the routing table 3b to obtain an output-destination datagram VCC that corresponds to a destination terminal address indicated by the header of the entered IP packet and notifies the ATM switch 2b to perform the switching of one packet.
In this datagram approach involving a higher layer (the IP layer), it is required that the above-mentioned processing operations (2) and (3) be performed whenever a cell arrives from the datagram VCC. An additional problem is that assembling the packet in step (2) above takes time.
Another problem is that the bandwidth of the line is exceeded or that the quality of the communication service cannot be assured. FIG. 37 is a diagram useful in describing this problem. This illustrates a case in which data is transmitted from terminal A to terminal E and from terminal B to terminal F by the datagram method (connectionless communication). An output-destination datagram VCC (VCC4) has been stored in the routing table 3b of the exchange EX1 in correspondence with the terminal addresses of the terminating-side terminals E, F, as shown in FIG. 36. Accordingly, a cell destined for terminal E arriving from the terminal A via the datagram VCC (VCC1) is sent to the output-destination datagram VCC (VCC4). Similarly, a cell destined for terminal F arriving from the terminal B via the datagram VCC (VCC2) is sent to the datagram VCC (VCC4). Though not shown, all datagram VCCs (VCC4) are allocated to cells addressed to terminals accommodated by the exchange EX2 from other terminals accommodated by the exchange EX1. Consequently, the bandwidth of the line LNG is exceeded or the quality of the communication service cannot be assured.
More specifically, since the datagram scheme requires IP layer switching at each exchange, a problem which arises is that the high-speed capability of the ATM switch cannot be manifested fully. Further, with the datagram scheme, one datagram VCC (VCC4) established between the exchanges EX1 and EX2 (FIG. 37) is used arbitrarily by a plurality of connectionless communication operations at the same time, as a consequence of which communication quality and the bandwidth necessary for persistence of communication cannot be assured. This means that this method is not suited to communication at a constant bit rate, which requires a real-time capability.
The cut-through method using an ATM switching unit is available as a solution to the above-mentioned problem encountered with the datagram method. FIG. 38A is a diagram useful in describing cut-through. Shown in FIG. 38A are the exchanges (IP switches) EX1xcx9cEX3, the ATM switch unit 2 and the IP router 3. Ordinarily, an IP packet is assembled from ATM cells that have arrived via the datagram VCC (shared VCC) and switching is performed based upon the destination IP address, as described earlier with reference to FIG. 36. With cut-through, however, a special-purpose ATM connection (VCC) between terminals A and E is established between the IP switches EX1, EX2 during the course of communication, and communication between the terminals A and E is performed via this VCC. As a result, route selection by the IP router 3 is made unnecessary and cut-through (a short cut) via the ATM switch unit 2 is achieved to realize higher speed.
An opportunity to use cut-through is when a terminal sends a packet required by an RSVP (resource reservation Protocol). The RSVP is a higher layer protocol than the Internet Protocol and demands bandwidth necessary for continuation of communication. By adopting this approach, the IP switch (exchange) is capable of ascertaining the required bandwidth and establishes the special-purpose VCC between the terminals A and E at the required bandwidth, thereby assuring the acquisition of the bandwidth necessary for continuation of communication. In other words, as shown in FIG. 38B, (1) switching usually is performed via the IP router. (2) When a packet demanded by the RSVP is detected, the required bandwidth is verified and the short-cut VCC is established at this bandwidth.
However, cut-through is merely an attempt to speed-up processing by the IP switch and assure the necessary bandwidth; it does not necessarily make possible the efficient utilization of network resources. For example, even if the IP switch is cut through, the route is still that decided by the IP router. When viewed from the end-to-end terminals, this route is not necessarily the shortest. In addition, in a case where the necessary bandwidth cannot be acquired with the route decided by the IP router, another route cannot be selected.
Accordingly, an object of the present invention is to make possible high-speed connectionless communication utilizing fully the high-speed switching characteristics of an exchange, e.g., an ATM exchange, for connection-oriented communication.
Another object of the present invention is to make possible datagram-type connectionless communication by inserting a datagram cell (leading cell), which indicates the address of the destination terminal, the necessary bandwidth desired by the terminal and the quality of service, at the head of a cell stream.
Another object of the present invention is to arrange it so that a network decides the shortest route and acquires the network resources necessary for persistence of communication and maintenance of communication quality while communication is being supported over route that has been decided by the datagram method.
A further object of the present invention is to arrange it so that a variety of protocols are handled by making it possible to identify which protocol address is specified by an address indicated by a datagram cell (leading cell).
Yet another object of the present invention is to notify a network of the latest required particulars and carry out communication conforming to the latest required particulars in a case where the necessary bandwidth and QOS (Quality of Service) class required by a terminal are changed during the course of communication.
Still another object of the present invention is to restore an assigned VCC to a not-in-use state in response to the end of communication (connectionless communication) based upon the datagram approach, thereby making it possible to utilize this VCC again for the purpose of another connectionless communication operation.
Another object of the present invention is to so arrange it that when a physical line which includes an assigned VCC does not have enough surplus bandwidth to satisfy a bandwidth of the same class, communication which satisfies the necessary bandwidth and QOS demanded by a terminal can be performed over a direct route separate from a communication route based upon the datagram approach.
Another object of the present invention is to so arrange it that when persistence (duration) of communication is managed and a datagram VCC for which persistence exceeds a certain threshold value exists, communication which meets the requirements demanded by a terminal can be performed over a direct route separate from a communication route based upon the datagram approach, thereby preventing lack of standby datagram VCCs in order to perform communication that continues over an extended period of time.
A further object of the present invention is to avoid duplication of direct VCC establishment requests to a network management center by arranging is so that an exchange which issues a direct VCC establishment request to the network management center is made only the exchange that accommodates the originating terminal.
A further object of the present invention is to lighten the load upon the originating terminal and suppress an increase in traffic in datagram-type multicast communication.
In accordance with the present invention, the foregoing objects are attained by providing a connectionless communication method for the connectionless transmission of data, comprising the steps of (1) establishing a plurality of VCCs (Virtual Channel Connections), each of which is exclusively for connectionless transfer of data, between mutually adjacent exchanges beforehand, (2) assigning one of the VCCs exclusively for connectionless data communication between terminals, and (3) performing connectionless data communication using this one VCC.
In accordance with the present invention, the foregoing objects are attained by providing a connectionless communication method for the connectionless transmission of data which further comprises the steps of (4) causing a terminal to disassemble connectionless data into data cells and then send the data cells, (5) causing the terminal to insert a cell (leading cell), which indicates a destination terminal address of a terminal that is the destination of the data, at the head of the data cells and send the leading cell prior to the data cells, (6) causing an exchange to assign a prescribed VCC for communication of the above-mentioned connectionless data upon referring to the destination terminal address contained in the leading cell, and (7) causing the exchange to transmit data cells, which have line identifier identical with that of the leading cell, using the VCC that has been assigned.